Apparatus and method for controlling slot usage

ABSTRACT

Provided is slot use control apparatus and method, including an access point (AP) to control a slotted channel access of a station (STA) in a wireless local area network (WLAN) includes generating a synchronization (synch) frame including an identification value indicating an STA allocated to a slot, and broadcasting the generated synch frame when a channel is in an idle state at a start point of the slot.

CROSS-REFERENCE TO THE RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 15/717,535, filed Sep. 27, 2017, which is a continuation applicationof U.S. application Ser. No. 14/412,669, filed Jan. 2, 2015, now U.S.Pat. No. 9,807,804, which is a national stage entry filed under 35U.S.C. § 371, of International Patent Application No. PCT/KR2013/005994,filed on Jul. 5, 2013, which claims the priority of U.S. provisionalapplication No. 61/746,070, the contents of which are incorporated byreference.

TECHNICAL FIELD

The present invention relates to an apparatus and method in which anaccess point (AP) managing a network controls a slotted channel accessof a station (STA) in a wireless local area network (WLAN) environment,and a terminal for operating thereof.

BACKGROUND ART

In a wireless local area network (WLAN), an excessive number of stations(STAs) in a network or a node may increase a probability of collisionsoccurring between the STAs. To solve such an issue, a method of using aslotted channel access may be proposed. In the method, an access point(AP) may divide a channel access interval into a slot having apredetermined length, and allocate, to the slot, a point in time atwhich the STA accesses the channel. The AP may notify the STA of alocation and an interval of the allocated slot using a beacon or abroadcast frame and thus, the STA may perform a data exchange in theallocated slot by listening to the beacon. Allocating a slot may allow apredetermined STA or a predetermined STA group exclusive access to achannel. In this instance, a disallowed STA may not access the channeland thus, a number of STAs simultaneously accessing the channel may bereduced. However, when an STA which does not listen to a beacon or apredetermined STA of a basic service set (BSS) adjacent to anoverlapping basic service set (OBSS) accesses the channel in theallocated slot, competition may occur with an STA already allocated tothe slot, because in the allocated slot, the STA may access the channelusing a distributed coordination function (DCF) of a carrier sensemultiple access with collision avoidance (CSMA/CA) scheme, in lieu of atime division multiple access (TDMA) scheme. In particular, when theallocated slot corresponds to an uplink (UL) for transmitting data tothe AP, a back-off occurring due to a collision between the STAs mayincrease in power consumption. To ensure that the STA allocated based onthe DCF accesses the channel in view of priority, the AP may need toadditionally protect the slot allocated to the STA. The STA may not makea full use of the allocated slot being protected with a method of, forexample, acquiring a network allocation vector (NAV) by a request tosend/clear to send (RTS/CTS) exchange, restricting the channel accessusing the beacon by the AP, or using a predetermined frame for theprotection. In this instance, another STA may be allowed to access thechannel by releasing the NAV.

DISCLOSURE OF INVENTION Technical Solutions

According to an aspect of the present invention, there is provided aslot use control method for an access point (AP) to control a slottedchannel access of a station (STA) in a wireless local area network(WLAN), the method including generating a synchronization (synch) frameincluding an identification value indicating an STA allocated to a slot,and broadcasting the generated synch frame when a channel is in an idlestate at a start point of the slot.

According to another aspect of the present invention, there is alsoprovided a slot use control apparatus for an AP to control a slottedchannel access of an STA in a WLAN, the apparatus including a generatorto generate a synch frame including an identification value indicatingan STA allocated to a slot, and a transmitter to broadcast the generatedsynch frame when a channel is in an idle state at a start point of theslot.

Effect of the Invention

According to an aspect of the present invention, it is possible toreduce power consumption by preventing an occurrence of a collisionbetween stations (STAs) and decrease a length of a delay in transmissioncaused by waiting.

When the STA does not use a slot, or when the slot has a remaining timeafter being used, another STA may be allowed to use the slot. Thus, anefficiency of an entire network resource may increase.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a wireless local area network (WLAN)environment including an access point (AP) and a plurality of terminals,for example, stations (STAs) to which an embodiment of the presentinvention is applied.

FIGS. 2 and 3 are diagrams illustrating an example of using asynchronization (synch) frame for avoiding an occurrence of a collisionand saving power when two STAs waking up from a sleeping state in aconventional slotted channel access scheme are a hidden node to eachother.

FIG. 4 is a flowchart illustrating the slot use control method accordingto an embodiment of the present invention.

FIG. 5 is a diagram illustrating an example of using a duration value inthe slot use control method according to an embodiment of the presentinvention.

FIGS. 6 and 7 are diagrams illustrating an operation of releasing a slotin the slot use control method according to an embodiment of the presentinvention.

FIG. 8 is a diagram illustrating an operation of supporting aschedule-negotiated station in the slot use control method according toan embodiment of the present invention.

FIG. 9 is a diagram illustrating a configuration of the slot use controlapparatus according to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described in detail with reference tothe accompanying drawings.

When it is determined detailed description related to a related knownfunction or configuration they may make the purpose of the presentinvention unnecessarily ambiguous in describing the present invention,the detailed description will be omitted here. Also, terminologies usedherein are defined to appropriately describe the exemplary embodimentsof the present invention and thus may be changed depending on a user,the intent of an operator, or a custom. Accordingly, the terminologiesmust be defined based on the following overall description of thisspecification.

FIG. 1 is a diagram illustrating a wireless local area network (WLAN)environment including an access point (AP) and a plurality of terminals,for example, stations (STAs) to which an embodiment of the presentinvention is applied.

In a WLAN defined by the Institute of Electrical and ElectronicsEngineers (IEEE) 802.11 standard, an environment in which a networkincludes an excessive number of STAs or a probability of collisionsoccurring between STAs due to a hidden node may be provided. To solvethis issue, a method of applying a slotted channel access scheme may beused in the environment. In the method, an AP 100 may divide a channelaccess interval into a slot having a predetermined length and allocate,to the slot, a point in time point at which an STA accesses a channel.

A core principle of the method is to reduce a number of STAssimultaneously accessing a channel by varying a channel access time foreach STA. To this end, the AP 100 may notify the STA of a location andan interval of the allocated slot using a beacon or a broadcast frame sothat the STA may listen to the beacon and perform a data exchange in theallocated slot. In this instance, since a disallowed STA may not accessthe channel, the number of STAs simultaneously accessing the channel maybe reduced.

However, when an STA accessing a channel without listening to a beaconor a predetermined STA of a basic service set (BSS) adjacent to anoverlapping basic service set (OBSS) accesses the channel in theallocated slot, competition may occur with an STA already allocated tothe slot, because, in the allocated slot, the STA may access the channelusing a distributed coordination function (DCF) based on a carrier sensemultiple access with collision avoidance (CSMA/CA) scheme, in lieu ofusing a time division multiple access (TDMA) scheme. For example, the AP100 may allocate a predetermined slot to a station group including anSTA-2 120 and an STA-3 130 and broadcast a location and an interval ofthe slot allocated to the station group including the STA-2 120 and theSTA-3 130. However, when an STA-1 110 fails to receive a broadcastframe, the STA-1 110 may try to access the slot allocated to the stationgroup. In this case, competition may occur, in the corresponding slot,between the STA-1 110 and the station group including the STA-2 120 andthe STA-3 130.

When the allocated slot corresponds to an uplink (UL) for transmittingdata to the AP 100, a back-off may occur due to a collision between theSTAs, which may lead to power consumption for reception resulting fromchannel listening and power consumption for transmission being greaterthan the power consumption for reception. Thus, reducing an occurrenceof the collision between the STAs is required.

Hereinafter, a method of reducing power consumption of the STA which isallocated using the conventional slotted channel access scheme will bedescribed with reference to FIGS. 2 and 3.

FIGS. 2 and 3 are diagrams illustrating an example of using asynchronization (synch) frame for avoiding an occurrence of a collisionand saving power when two STAs waking up from a sleeping state in aconventional slotted channel access scheme are a hidden node to eachother.

An STA that wakes up at a slot start by receiving an allocated slot froman AP may be unaware of the existence of the hidden node. To assist withthis, the AP may transmit the synch frame when a channel is in an idlestate at the slot start.

Referring to FIG. 1, in the conventional slotted channel access scheme,the STA may receive a synch frame 202 from the AP at a slot boundary 201and access a channel using a DCF. For example, in 210, the STA in anawake state may receive a beacon message from the AP. In 220, the STAmay wake up at the slot boundary 201 and wait for a channel synch. Inthis instance, the AP may transmit the synch frame 202 from the slotboundary 201 to the STA. In 230, the STA may synch to the channel usingthe synch frame 202 received from the AP, and start a channel accessaccording to DCF rules.

As another conventional operation example, referring to FIG. 3, when anAP senses the channel is busy or is in a process of receiving data fromthe STA at a slot boundary 301, the AP may not transmit the synch framein 302. Since the STA does not receive the synch frame, the STA may waitwithout accessing the channel. According to a waiting rule of an STA,the waiting may be continued until one of (i) receiving the synch, (ii)receiving another frame from the access point, and (iii) ending of Probedelay interval is satisfied.

Accordingly, the STA, which wakes up at the allocated slot boundary 301and tries to access the channel, may be unaware whether thecorresponding channel is occupied. Thus, the STA may determine whetherthe corresponding channel is occupied using the synch frame transmittedfrom the AP, and synch to a medium by receiving the synch frame toaccess the channel.

For example, in 310, when an STA-x is a hidden node with respect to anSTA-n, the STA-x may be in a process of transmitting a packet to the APcrossing the slot boundary 301. In 320, the STA may wake up at the slotboundary 301 and wait for the packet to synch to the medium. Here, theSTA may not listen to a data packet from the STA-x. In 330, the STA maysynch to the channel by receiving an ACK frame from the AP.

In a conventional method of FIGS. 5 and 6, an operation of an allocatedSTA is explained without describing operations of another STA in anetwork. The other STA, aside from the allocated STA, may also wake upand initiate a channel access by receiving a synch frame. In thisinstance, an operation of an STA may vary depending on whether an accessof the STA allocated to a slot is to be allowed or an access of all STAsreceiving the synch frame is to be allowed. Thus, predeterminations withrespect to the operation of the allocated STA and the operation of theother STA in a network are equally required.

FIG. 4 is a flowchart illustrating the slot use control method accordingto an embodiment of the present invention.

Referring to FIG. 4, an AP may control a slotted channel access of anSTA in a WLAN. Hereinafter, a method of controlling the slotted channelaccess of an STA by the AP is referred to as the slot use controlmethod.

In operation 410, a synch frame indicating an STA allocated to a slotmay be generated using the slot use control method. The STA allocated toa slot may be a single STA or a station group including a plurality ofSTAs.

Depending on a case, the synch frame may be generated by setting areceiver address (RA) value in a media access control (MAC) header.

In the slot use control method, when a short MAC (SM) header is used toreduce a scale of the MAC header, a receiver association identification(AID) value and a basic service set identification (BSSID) value may beused in lieu of the RA value. For example, the synch frame may begenerated by setting the receiver AID value and the BSSID value to theSM header. In this instance, since an AID is unique to a basic serviceset (BSS), an STA may be uniquely identified through concurrent use of aBSSID.

When the station group is hierarchically divided using the AID value,the station group may be identified based on most significant bits(MSBs) of the AID used to identify a group. For example, when the STAallocated to a slot corresponds to the station group including aplurality of STAs and the station group is hierarchically divided usingthe AID value, the synch frame may be generated using a value of MSBs ofthe AID value as an identification value in the slot use control method.

Depending on an example, least significant bits (LSBs) may be allowed touse one pattern. In the slot use control method, a value of LSBs of theAID value may have identical patterns. For example, all the LSBs may beset to “0”.

However, since the station group may include an STA having a combinationof MSBs and LSBs, a separate bit field may be prepared to identify agroup station and a unique station. In the slot use control method, whenthe STA allocated to a slot corresponds to the group station including aplurality of STAs, the synch frame may be generated by setting anidentification bit field to identify the group station and the uniquestation included in the station group.

Since the synch frame is transmitted to each frame, having a shorterlength of the synch frame may be advantageous. Thus, the synch frame maybe provided in a format of a null data packet (NDP). The NDP may be aframe including a preamble. The preamble may include a long trainingfield (LTF), a short training field (STF), and a signal (SIG) field forsynchronization and channel estimation. Thus, the receiver AID and theBSSID may be applied to the SIG field. In the slot use control method,the synch frame may be generated in a format including at least one ofthe LTF, the STF, and the SIG field. In the slot use control method, thereceiver AID value and the BSSID value may be set in the SIG field forsetting of the synch frame.

In this instance, a partial AID or a partial BSSID may be used due to alimitation on a number of bits in the SIG field. Also, a form ofscrambling may be used because a unique determination may not beperformed with the partial AID or the partial BSSID. Thus, the partialAID value or the partial BSSID value may be set in the SIG field in theslot use control method.

As described above, when the synch frame including the identificationvalue is generated and when the channel is in an idle state at a startpoint of the slot, the generated synch frame may be broadcast inoperation 420. Thus, when the synch frame is received, the STA indicatedin the synch frame may be allowed a channel access. Other STAs which aredisallowed the channel access may wait.

Depending on an example, a duration value may be notified for savingpower of other STAs aside from an indicated STA in the slot use controlmethod. Hereinafter, an example of using the duration value will bedescribed with reference to FIG. 5.

FIG. 5 is a diagram illustrating an example of using a duration value inthe slot use control method according to an embodiment of the presentinvention.

Referring to FIG. 5, in operation 510, an AP 500 may generate a synchframe to further include the duration value. In the slot use controlmethod, for example, the synch frame may be generated to include theduration value and an identification value indicating an STA 501allocated to a slot. In operation 520, when the synch frame is received,the STA 501 indicated in the synch frame may be allowed a channelaccess.

In operation 530, other STAs 502, aside from the STA 501 indicated inthe synch frame, may set a network allocation vector (NAV) using areceived duration value. In operation 540, the other STAs 502 may notaccess a channel until the NAV corresponds to “0”. For example, theother STAs 502 may be changed into a sleep status until the NAVcorresponds to “0”. Thus, the STA 501 allocated to the slot may beprotected from an occurrence of collisions.

As an example, when an NDP is used, a duration field may not be addeddue to a limitation on a number of bits. In this example, a length of aninterval may be indicated using a value determined implicitly. The synchframe may be transmitted in each of the intervals. In the slot usecontrol method, a value indicating a length of an interval in which thesynch frame is retransmitted may be set. Also, the synch frame may beretransmitted in each of the intervals having the predetermined value.

An interval value generally has the same meaning as a meaning of a slotduration. Since the interval value is used for an STA which does notlisten to a beacon or an STA of an OBSS, the interval value may be afixed value rather than a varying value, such as, the slot duration. Forexample, the value indicating a length of an interval in which the synchframe is retransmitted may be set to the fixed value. Transmitting thesynch frame in each of fixed intervals is to protect a slot from beingused by an STA entering a channel in an allocated slot interval.

According to an example embodiment of the present invention, a slotbeing protected may be released so that another STA may be allowedaccess. Hereinafter, an operation of releasing a slot will be describedwith reference to FIGS. 6 and 7.

FIGS. 6 and 7 are diagrams illustrating an operation of releasing a slotin the slot use control method according to an embodiment of the presentinvention.

Protection of the allocated slot may be implemented using one ofacquiring an NAV by an RTS/CTS exchange, restricting access using abeacon by an AP, setting the NAV using a synch frame including aduration field, and the like, or a combination thereof.

An STA allocated to the slot may not make a full use of a slot beingprotected. In this instance, a CF-end frame may be used to allow anaccess of another STA by releasing an NAV in the slot use control methodaccording to an embodiment.

Referring to FIG. 6, when an STA 601 allocated to a slot being protecteddoes not enter, the slot may be released in the slot use control method.

Referring to FIG. 6 again, in a case of a UL, an AP 600 may transmit asynch frame in operation 610. Also the AP 600 may wait for a frame fromthe STA 601 allocated to the slot to be received.

In operation 620, when the frame is not received from the STA 601allocated to the slot during a period of data sensing time 603, theCF-end frame may be transmitted in the slot use control method. Forexample, the AP 600, by transmitting the CF-frame, may notify anotherSTA 602 aside from the STA 601 allocated to the slot, of release of theallocated slot

In operation 630, the other STA 602 received the CF-end frame may reseta predetermined NAV. In operation 640, the other STA 602 may access achannel. In this instance, the other STA 602 may start the channelaccess after recognizing a duration field value included in the CF-endframe.

In a case of a downlink (DL), the AP 600 may transmit a requestmanagement frame to the STA 601 allocated to the slot in operation 610.In operation 620, when a response management frame in response to therequest management frame is not received from the STA 601 allocated tothe slot during a predetermined period of the data sensing time 603, theAP 600 may be aware that an STA is absent and transmit the CF-end frame.Here, the response management frame may be, for example, an ACK framewith respect to data transmitted to the STA 601 allocated to the slot, aCTS frame pertaining to an RTA, a response management frame pertainingto a management frame having a format of a request, and the like. Anoperation of the other STA 602 received the CF-frame may be identicallyperformed to the case of the UL.

Referring to FIG. 7, when data to be subsequently used is absent duringuse of a channel, an STA 701 allocated to a slot may release thechannel.

In operation 710, when a frame to be subsequently transmitted is absent,the STA 701 allocated to a slot may transmit a CF-end frame to an AP700. Depending on a case, the STA 701 may set a value including a sum ofa time for transmitting a CF-end frame, a short interframe space (SIFS),and a time for transmitting a response CF-end frame as a duration fieldvalue.

In operation 720, the AP 700 may receive the CF-end frame from the STA701 allocated to the slot and set the duration field value as “0”.

In operation 730, the AP 700 may transmit the CF-end frame to which theduration field value is set to “0”, to the other STA 702 aside from theSTA 701 allocated to the slot.

In operation 740, when the other STA 702 receives at least one of theCF-frame transmitted from the STA 701 allocated to the slot and theCF-end frame transmitted from the AP 700, the other STA 702 may accessthe channel after a channel duration time ends.

According to an embodiment of the present invention, a method of NAVsetting by using the duration field and a method of transmitting thesynch frame for each predetermined interval during a data exchange maybe applied to a resource allocated by being scheduled without using aslotted channel access. Hereinafter, a further description will beprovided with respect to FIG. 8.

FIG. 8 is a diagram illustrating an operation of supporting aschedule-negotiated station in the slot use control method according toan embodiment of the present invention.

An AP may support an STA that wakes up based on a negotiated scheduleand tries to perform a data exchange. The STA may have a low-duty cycle,for example, a sensor, and be sensitive to power consumption. Thus, theSTA may perform the data exchange by waking up at a scheduled time. TheAP may notify a predetermined STA of time information after anallocation is performed. Here, the time information may include a targetwake time (TWT) indicating a time to wake up, and an interval indicatingan allocated time interval after waking up. Similar to theaforementioned case of the slotted channel access, when the STA wakes upat the TWT, a collision may occur due to a hidden node.

For example, the collision may occur when the other STA wakes up whilethe STA is in a process of transmitting data, or when an OBSS STA, whichis a hidden node with respect to the STA transmitting data, is present.Since a significant reduction in power consumption of the STA isrequired to avoid an occurrence of the collision, the AP may transmitthe synch frame in a middle of data, and transmit a duration fieldincluding a remaining time for an allocated time interval.

In operation 810, an STA may wake up at a TWT 801 and wait for a channelsynch. In operation 820, to assist the STA to rapidly synch to achannel, the AP may transmit the synch frame at the TWT 801 when thechannel is in an idle state. In operation 830, the STA may synch to thechannel by receiving the synch frame, and start a channel accessaccording to enhanced distributed channel access (EDCA) rules. Inoperation 840, when data transmission ends, the STA may revert to asleep mode.

FIG. 9 is a diagram illustrating a configuration of a slot use controlapparatus according to an example embodiment.

Referring to FIG. 9, a slot use control apparatus 900 may control aslotted channel access of an STA in a WLAN. The slot use controlapparatus 900 may be disposed in an AP to be operated.

The slot use control apparatus 900 includes a generator 910 and atransmitter 920.

The generator 910 may generate a synch frame including a valueindicating an STA allocated to a slot. The STA allocated to a slot maybe a single STA and a station group including a plurality of STAs.

Depending on a case, the synch frame may be generated by setting an RAvalue in an MAC header.

When an SM header is used to reduce a scale of the MAC header, areceiver AID value and a BSSID value may be used in lieu of an RA value.For example, the synch frame may be generated by setting the receiverAID value and the BSSID value to the SM header in the slot use controlmethod. In this instance, since an AID is unique to a BSS, an STA may beuniquely identified through concurrent use of a BSSID.

When the station group is hierarchically divided using the AID value,the station group may be identified based on MSBs of the AID used toidentify a group. For example, when the STA allocated to a slotcorresponds to the station group including a plurality of STAs and thestation group is hierarchically divided using the AID value, thegenerator 910 may generate the synch frame using a value of MSBs of theAID value as an identification value.

Depending on an example, LSBs may be allowed to use one pattern. Interms of the generator 910, a value of LSBs of the AID value may haveidentical patterns. For example, the value of LSBs may be set to “0”.

However, since the station group may include an STA having a combinationof MSBs and LSBs, a separate bit field may be prepared to distinguish agroup station and a unique station. When the STA allocated to a slotcorresponds to the group station including a plurality of STAs, thegenerator 910 may generate the synch frame by setting an identificationbit field to identify the group station and the unique STA included inthe station group.

Since the synch frame is transmitted to each frame, having a shorterlength of the synch frame may be advantageous. Thus, the synch frame maybe provided in a form of an NDP. Here, the NDP may be a frame includinga preamble. The preamble may include an LTF, an STF, and an SIG fieldfor synchronization and channel estimation. Thus, the receiver AID andthe BSSID may be applied to the SIG field. For example, the generator910 may generate the synch frame to have a format including at least oneof the LTF, the STF, and the SIG field. The generator 910 may also setthe synch frame by setting the receiver AID value and the BSSID value tothe SIG field.

In this instance, a partial AID or a partial BSSID may be used due to alimitation on a number of bits in the SIG field. Also, a form ofscrambling may be used because a unique determination may not beperformed with the partial AID or the partial BSSID. Thus, the generatormay set the partial AID value or the partial BSSID value to the SIGfield.

As described above, when the synch frame including the identificationvalue is generated and the channel is in an idle state at a start pointof the slot, the transmitter 920 may broadcast the generated synchframe. Thus, when the synch frame is received, the STA indicated in thesynch frame may be allowed a channel access. Other STAs which aredisallowed the channel access may wait.

Depending on an example, the generator 910 may add a duration value forsaving power of STAs, aside from an indicated STA.

For example, the generator 910 may generate a synch frame to include theduration value and an identification value indicating an STA allocatedto a slot. When the synch frame is received, the STA indicated in thesynch frame may be allowed a channel access.

The other STAs, aside from the STA indicated in the synch frame, may setan NAV using a received duration value. Also, the other STAs may notaccess a channel until the NAV corresponds to “0”. For example, theother STAs may be changed into a sleep status until the NAV correspondsto “0”. Thus, the STA allocated to the slot may be protected from anoccurrence of collisions.

As an example, when an NDP is used, a duration field may not be addeddue to a limitation on a number of bits. In this example, a length of aninterval may be indicated using a value determined implicitly. The synchframe may be transmitted in each of the intervals. The generator 910 mayset a value indicating a length of an interval in which the synch frameis retransmitted. Also, the generator 910 may retransmit the synch framefor each of the intervals having the predetermined value.

An interval value generally has the same meaning as a meaning of a slotduration. Since the interval value is used for an STA which does notlisten to a beacon or an OBSS STA, the interval value may use a fixedvalue rather than a varying value, such as, the slot duration. Forexample, the generator 910 may set the value indicating a length of aninterval in which the synch frame is retransmitted, as the fixed value.Transmitting the synch frame in each of fixed intervals is to protect aslot use from an STA entering a channel in an allocated slot interval.

According to an embodiment of the present invention, the slot usecontrol apparatus may release a slot being protected from the protectionso that another STA may be allowed access.

Protection of the allocated slot may be implemented using one ofacquiring an NAV by an RTS/CTS exchange, restricting an access using abeacon by an AP, setting the NAV using a synch frame including aduration field, and the like, or a combination thereof.

An STA allocated to the slot may not make a full use of a slot beingprotected. In this instance, the slot use control apparatus 900according to an example embodiment may use a CF-end frame to allow anaccess of the other STAs by releasing the NAV.

In a case of an UL, the transmitter 902 included in the slot use controlapparatus 900 may transmit the synch frame. Also the slot use controlapparatus 900 may wait for receiving a frame from the STA allocated tothe slot.

When the frame is not received from the STA allocated to the slot duringa period of data sensing time, the generator 910 may be transmit theCF-end. For example, the slot use control apparatus 900, by transmittingthe CF-frame, may notify the other STA aside from the STA allocated tothe slot, of release of the allocated slot

The other STA received the CF-end frame may reset a predetermined NAV.Also, the other STA may access a channel. In this instance, the otherSTA may start the channel access after recognizing a duration fieldvalue included in the CF-end frame.

In a case of a DL, the transmitter 920 may transmit a request managementframe to the STA allocated to the slot. When a response management frameresponding to the request management frame is not received from the STAallocated to the slot during a predetermined period of the data sensingtime, the slot use control apparatus 900 may be aware that an STA isabsent, and transmit the CF-end frame. Here, the response managementframe may be, for example, an ACK frame with respect data transmitted tothe STA allocated to the slot, a CTS frame with respect to an RTA, aresponse management frame with respect to a management frame in a formof request, and the like. An operation of the other STA received theCF-frame may be performed identically to the case of the UL.

Depending on an example, when data to be subsequently used is absentduring use of the channel, the STA allocated to a slot may release thechannel.

When a frame to be subsequently transmitted is absent, the STA allocatedto a slot may transmit the CF-end frame to the slot use controlapparatus 900. As an example, the STA may set a value including a sum ofa time for transmitting the CF-end frame, an SIFS, and a time fortransmitting a response CF-end frame as a duration field value.

The slot use control apparatus 900 may receive the CF-end frame from theSTA allocated to the slot and set the duration field value as “0”.

Also, the transmitter 920 may transmit the CF-end frame to which theduration field value is set as “0” to the other STA aside from the STAallocated to the slot.

When the other STA receives at least one of the CF-frame transmittedfrom the STA allocated to the slot and the CF-end frame transmitted fromthe AP, may access the channel after a channel duration time ends.

The method according to the above-described embodiments may be recordedin non-transitory computer-readable media including program instructionsto implement various operations embodied by a computer. The media mayalso include, alone or in combination with the program instructions,data files, data structures, and the like. Examples of non-transitorycomputer-readable media include magnetic media such as hard disks,floppy disks, and magnetic tape; optical media such as CD ROM discs andDVDs; magneto-optical media such as optical discs; and hardware devicesthat are specially configured to store and perform program instructions,such as read-only memory (ROM), random access memory (RAM), flashmemory, and the like. Examples of program instructions include bothmachine code, such as produced by a compiler, and files containinghigher level code that may be executed by the computer using aninterpreter. The described hardware devices may be configured to act asone or more software modules in order to perform the operations of theabove-described embodiments, or vice versa.

Although a few embodiments of the present invention have been shown anddescribed, the present invention is not limited to the describedembodiments. Instead, it would be appreciated by those skilled in theart that changes may be made to these embodiments without departing fromthe principles and spirit of the invention, the scope of which isdefined by the claims and their equivalents.

The invention claimed is:
 1. A method for an access point (AP) toprovide synchronization to a station (STA) in a wireless local areanetwork (WLAN), the method comprising: transmitting a synchronizationframe including a receiver address (RA) field including anidentification value indicating a STA; and after transmitting thesynchronization frame, waiting for a response waiting time; when aresponse frame in response to the synchronization frame is not receivedduring the response waiting time, transmitting a contention free-end(CF-End) frame, wherein the CF-End frame is transmitted by the AP toreset a network allocation vector (NAV), and wherein the synchronizationframe is transmitted when a channel is in an idle state at the timepoint when the STA wakes up.
 2. The method of claim 1, wherein thesynchronization frame is transmitted in a form of a null data packet(NDP) which includes a Short Training Field (STF) field, a Long TrainingField (LTF) field, a Signal (SIG) field without a Data field.
 3. Themethod of claim 1, wherein the RA field of the synchronization framecomprises information of at least one of a partial associationidentifier (AID) or a partial basic service set identifier (BSSID). 4.The method of claim 1, wherein a time point when the STA wakes upcorresponds to at least one of a slot boundary of time slots allocatedto the STA or a target wake time (TWT) for the STA.
 5. The method ofclaim 4, the synchronization frame is used for protecting at least oneof the time slot allocated to the STA or the TWT for the STA.
 6. Themethod of claim 1, wherein the response frame is transmitted from theSTA to the AP.
 7. The method of claim 6, wherein the response frame isan uplink data frame.
 8. The method of claim 1, wherein thesynchronization frame further includes a Duration field.
 9. The methodof claim 8, a value of the Duration field is used for transmitting theresponse frame by the STA.
 10. The method of claim 8, a value of theDuration field is used for setting a NAV by at least one station otherthan the STA.
 11. An apparatus for an access point (AP) to provide asynchronization to a station (STA) in a wireless local area network(WLAN), the apparatus comprising: a generator configured to generate asynchronization frame including a receiver address (RA) field includingan identification value indicating a STA; and a transmitter configuredto transmit the synchronization frame, wherein, after transmitting thesynchronization frame, the apparatus waits for a response waiting time,wherein the transmitter further configured to transmit a contentionfree-end (CF-End) frame when a response frame in response to thesynchronization frame is not received during the response waiting time,wherein the CF-End frame is transmitted by the AP to reset a networkallocation vector (NAV), and wherein the synchronization frame istransmitted when a channel is in an idle state at the time point whenthe STA wakes up.
 12. A method for a station (STA) in a wireless localarea network (WLAN), the method comprising: receiving a synchronizationframe from an access point (AP), the synchronization frame including areceiver address (RA) field having an identification value indicatingthe STA; and receiving a contention free-end (CF-End) frame after aresponse frame in response to the synchronization frame is nottransmitted to the AP during a response time; wherein the CF-End frameis received from the AP to reset a network allocation vector (NAV), andwherein the synchronization frame is transmitted when a channel is in anidle state at the time point when the STA wakes up.